U.S. patent number 7,116,987 [Application Number 10/798,988] was granted by the patent office on 2006-10-03 for location estimation of wireless terminals through pattern matching of deduced and empirical signal-strength measurements.
This patent grant is currently assigned to Polaris Wireless, Inc.. Invention is credited to Tarun Kumar Bhattacharya, Robert Lewis Martin, David Stevenson Spain, Jr..
United States Patent |
7,116,987 |
Spain, Jr. , et al. |
October 3, 2006 |
Location estimation of wireless terminals through pattern matching
of deduced and empirical signal-strength measurements
Abstract
A method of estimating the location of a wireless terminal is
disclosed. The illustrative embodiment of the present invention is
based on the observation that the signal strength of a signal from
a transmitter is different at some locations, and, therefore, the
location of a wireless terminal can be estimated by comparing the
signal strength it currently observes against a map or database
that correlates locations to signal strengths. Furthermore, the
illustrative embodiment deduces the signal strength of one or more
base stations' control channels at the wireless terminal based on
the principal of reciprocity, whether or not the wireless terminal
can actually receive the base stations' control channels but so
long as the base station can receive and measure the uplink signal
from the wireless terminal. The deduced signal-strength
measurements can then used--alone or in combination with the
empirical signal-strength measurements--to estimate the location of
the wireless terminal.
Inventors: |
Spain, Jr.; David Stevenson
(Portola Valley, CA), Martin; Robert Lewis (Antioch, CA),
Bhattacharya; Tarun Kumar (San Jose, CA) |
Assignee: |
Polaris Wireless, Inc. (Santa
Clara, CA)
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Family
ID: |
34068465 |
Appl.
No.: |
10/798,988 |
Filed: |
March 12, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050014518 A1 |
Jan 20, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60488866 |
Jul 19, 2003 |
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Current U.S.
Class: |
455/456.1;
455/513; 455/456.5 |
Current CPC
Class: |
G01S
5/0252 (20130101); H04W 64/00 (20130101) |
Current International
Class: |
H04Q
7/20 (20060101) |
Field of
Search: |
;455/456.1,456.2,456.3,456.5,513,457,134,115.3,161.3,402,226.1,226.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tran; CongVan
Attorney, Agent or Firm: DeMont & Breyer, LLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of: i. U.S. Provisional Patent
Application No. 60/488,866, filed 19 Jul. 2003, and entitled
"Location Estimation of Wireless Terminals Through Pattern Matching
of Deduced Signal Strengths", which application is also
incorporated by reference.
The underlying concepts, but not necessarily the nomenclature, of
the following are incorporated by reference: i. U.S. Pat. No.
6,269,246, issued 31 Jul. 2001; ii. U.S. patent application Ser.
No. 09/532,418, filed 22 Mar. 2000; iii. U.S. patent application
Ser. No. 10/128,128, filed 22 Apr. 2002; iv. U.S. patent
application Ser. No. 10/299,398, filed 18 Nov. 2002; v. U.S. patent
application Ser. No. 10/357,645, filed 4 Feb. 2003; vi. U.S. patent
application Ser. No. 60/449,569, filed 24 Feb. 2003; and vii. U.S.
patent application Ser. No. 60/461,219, filed 8 Apr. 2003.
Claims
What is claimed is:
1. A method comprising: receiving a signal-strength measurement for
a first downlink signal that is received by a wireless terminal and
a signal-strength measurement for an uplink signal that is
transmitted by said wireless terminal; and estimating the location
of said wireless terminal based on said signal-strength measurement
for said first downlink signal and on said signal-strength
measurement for said uplink signal wherein estimating the location
of said wireless terminal comprises pattern matching (i) said
signal-strength measurement for said downlink signal against a map
that correlates signal-strength measurements and locations.
2. The method of claim 1 further comprising deducing the signal
strength of a second downlink signal at said wireless terminal
based on said signal-strength measurement for said uplink
signal.
3. The method of claim 2 wherein estimating the location of said
wireless terminal comprises pattern matching (ii) the deduced
signal strength of said downlink signal at said wireless terminal
against a map that correlates signal-strength measurements and
locations.
4. The method of claim 2 wherein estimating the location of said
wireless terminal comprises pattern matching the difference between
(i) said signal-strength measurement for said downlink signal and
(ii) the deduced signal strength of said downlink signal at said
wireless terminal against a map that correlates signal-strength
measurements and locations.
5. The method of claim 2 wherein deducing the signal strength of
said second downlink signal at said wireless terminal is also based
on the transmit strength of said second downlink signal.
6. The method of claim 2 wherein deducing the signal strength of
said second downlink signal at said wireless terminal is also based
on the transmit strength of said uplink signal.
7. The method of claim 1 further comprising transmitting a request
to a mobile switching center to make said signal-strength
measurement for said uplink signal.
8. A method comprising: receiving (i) a first signal-strength
measurement for an uplink signal that is transmitted by a wireless
terminal and received at a first location and (ii) a second
signal-strength measurement for said uplink signal that is
transmitted by said wireless terminal and received at a second
location; deducing the signal strength of a first downlink signal
at said wireless terminal based on said first signal-strength
measurement for said uplink signal; deducing the signal strength of
a second downlink signal at said wireless terminal based on said
second signal-strength measurement for said uplink signal; and
estimating the location of said wireless terminal based on the
deduced signal strength of said first downlink signal and on the
deduced signal strength of said second downlink signal.
9. The method of claim 8 wherein estimating the location of said
wireless terminal comprises pattern matching (i) the deduced signal
strength of said first downlink signal and (ii) the deduced signal
strength of said second downlink signal against a map that
correlates signal-strength measurements and locations.
10. The method of claim 8 wherein estimating the location of said
wireless terminal comprises pattern matching the difference between
(i) the deduced signal strength of said first downlink signal and
(ii) the deduced signal strength of said second downlink signal
against a map that correlates signal-strength measurements and
locations.
11. The method of claim 8 wherein deducing the signal strength of a
first downlink signal at said wireless terminal is based on the
transmit strength of said first downlink signal.
12. The method of claim 8 wherein deducing the signal strength of a
first downlink signal at said wireless terminal is based on the
transmit strength of said uplink signal.
13. The method of claim 8 further comprising transmitting a request
to a mobile switching center to make said first signal-strength
measurement for an uplink signal and said second signal-strength
measurement for said uplink signal.
14. The method of claim 8 further comprising receiving a message
from a mobile switching center that comprises said first
signal-strength measurement for an uplink signal and said second
signal-strength measurement for said uplink signal.
15. A method comprising: transmitting a request to a mobile
switching center to make a first signal-strength measurement at a
first location and a second signal-strength measurement at a second
location for an uplink signal that is transmitted by a wireless
terminal; receiving a message from said mobile switching center
that comprises said first signal-strength measurement and said
second signal-strength measurement; deducing the signal strength of
a first downlink signal at said wireless terminal based on said
first signal-strength measurement; deducing the signal strength of
a second downlink signal at said wireless terminal based on said
second signal-strength measurement; and estimating the location of
said wireless terminal based on the deduced signal strength of said
first downlink signal and on the deduced signal strength of said
second downlink signal.
16. The method of claim 15 wherein estimating the location of said
wireless terminal comprises pattern matching (i) the deduced signal
strength of said first downlink signal and (ii) the deduced signal
strength of said second downlink signal against a map that
correlates signal-strength measurements and locations.
17. The method of claim 15 wherein estimating the location of said
wireless terminal comprises pattern matching the difference between
(i) the deduced signal strength of said first downlink signal and
(ii) the deduced signal strength of said second downlink signal
against a map that correlates signal-strength measurements and
locations.
18. The method of claim 15 wherein deducing the signal strength of
a first downlink signal at said wireless terminal is based on the
transmit strength of said first downlink signal.
19. The method of claim 15 wherein deducing the signal strength of
a first downlink signal at said wireless terminal is based on the
transmit strength of said uplink signal.
Description
FIELD OF THE INVENTION
The present invention relates to telecommunications in general,
and, more particularly, to a technique for estimating the location
of a wireless terminal.
BACKGROUND
FIG. 1 depicts a map of a geographic region that is serviced by a
wireless telecommunications system, which system provides wireless
telecommunications service to wireless terminals (e.g., wireless
terminal 101) within the region. The heart of the
telecommunications system is wireless switching center 110, which
might also be known as a mobile switching center ("MSC") or a
mobile telephone switching office ("MTSO").
Typically, wireless switching center 111 is connected through a
plurality of intermediate network elements (e.g., base station
controllers, etc.) to a plurality of base stations (e.g., base
stations 102-1, 102-2, and 102-3), which are dispersed throughout
the geographic area serviced by the system. As depicted in FIG. 1,
base station 102-2 serves wireless terminal 101.
As is well known to those skilled in the art, wireless switching
center 111 is responsible for, among other things, establishing and
maintaining calls between wireless terminals and between a wireless
terminal and a wireline terminal (which is connected to the system
via the local and/or long-distance telephone networks and which are
not shown in FIG. 1).
The salient advantage of wireless telecommunications over wireline
telecommunications is the mobility that is afforded to the users of
the wireless telecommunications system. On the other hand, the
salient disadvantage of wireless telecommunications lies in that
fact that because the user is mobile, an interested party might not
be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless
terminal and remote parties. There are a variety of reasons why the
user of a wireless terminal might be interested in knowing his or
her own location. For example, the user might be interested in
telling a remote party where he or she is.
There are a variety of reasons why a remote party might be
interested in knowing the location of the user. For example, the
recipient of a 911 emergency call from a wireless terminal might be
interested in knowing the location of the wireless terminal so that
emergency services vehicles can be dispatched to that location.
There are many techniques in the prior art for estimating the
location of a wireless terminal.
In accordance with one technique, the location of a wireless
terminal is estimated to be at the centroid of the cell in which
the wireless terminal is located. This technique is advantageous in
that it does not require that additional hardware be added to the
wireless terminal or to the wireless telecommunications system, and
this means that the first technique can be inexpensively
implemented in legacy systems. The first technique is only
accurate, however, to a few kilometers, and, therefore, it is
generally not acceptable for applications (e.g., emergency services
dispatch, etc.) that require higher accuracy.
In accordance with a second technique, the location of a wireless
terminal is estimated by triangulating the angle of arrival or the
time of arrival of the signals transmitted by the wireless terminal
to be located at various receivers. This technique is accurate to
within a few hundreds of meters and is advantageous in that it can
be used with legacy wireless terminals. It is disadvantageous,
however, in that it generally requires that hardware be added to
the telecommunication system's base stations, and this is very
expensive.
In accordance with a third technique, the location of a wireless
terminal is estimated by a radio navigation unit, such as a Global
Positioning System (GPS) receiver, that is incorporated into the
wireless terminal. This technique is accurate to within tens of
meters and is advantageous in that it does not require that
additional hardware be added to the telecommunication system's
infrastructure. The third technique is disadvantageous, however, in
that it cannot be used with legacy wireless terminals that do not
comprise a radio navigation unit.
Therefore, the need exists for a technique for estimating the
location of a wireless terminal with higher resolution than the
first technique and that can be inexpensively implemented in legacy
systems.
SUMMARY OF THE INVENTION
The present invention enables the estimation of the location of a
wireless terminal without the addition of hardware to either the
wireless terminal or to the telecommunication system's base
stations. Some embodiments of the present invention are, therefore,
ideally suited for use with legacy telecommunications systems.
The illustrative embodiment of the present invention is based on
the observation that the signal strength of a signal from a
transmitter is different at some locations, and, therefore, the
location of a wireless terminal can be estimated by comparing the
signal strength it currently observes against a map or database
that correlates locations to signal strengths. For example, if a
particular radio station is known to transmit a strong signal to a
first location and a weak signal to a second location, and a given
wireless terminal at an unknown location is receiving the radio
station with a weak signal, it is more likely that the wireless
terminal is at the second location than it is at the first
location.
The accuracy of the estimate of the location of a wireless terminal
can be enhanced when the principle uses multiple transmitters and
multiple signals. A simplified example illustrates this point. A
first radio station, Radio Station A, transmits a strong signal to
Location 1 and Location 2, but a weak signal to Location 3 and
Location 4, and a second radio station, Radio Station B, transmits
a strong signal to Location 1 and Location 3, but a weak signal to
Location 2 and Location 4. This information is summarized in the
table below and forms the basis for a map or database that
correlates locations to signal strength.
TABLE-US-00001 TABLE 1 Illustrative Signal Strength Database
(Absolute Signal Strength) Radio Station A Radio Station B Location
1 Strong Signal Strong Signal Location 2 Strong Signal Weak Signal
Location 3 Weak Signal Strong Signal Location 4 Weak Signal Weak
Signal
If a given wireless terminal at an unknown location receives Radio
Station A with a weak signal and Radio Station B with a strong
signal, it is more likely that the wireless terminal is at Location
3 than it is at either Location 1, 2, or 4.
Furthermore, the accuracy of the estimate of the location of a
wireless terminal can be enhanced when the signal strength of each
signal at each location is quantified. A simplified example
illustrates this point. If a particular radio station is known to
be received in one location with a strength of -50 dBm, at a second
location with a strength of -53 dBm, and at a third location with a
strength of -55 dBm, then the reception of the signal with a
strength of -56 dBm suggests that the wireless terminal is more
likely at the third location than at either the first or second
location.
In the prior art, a wireless terminal measures the signal strength
of the control channels of the base stations that it can receive
and that are not serving it and reports some or all of those
signal-strength measurements back to the wireless switching center.
In the prior art this is performed so that the wireless switching
center can intelligently decide which base station the wireless
terminal should be served by. In accordance with the illustrative
embodiment of the present invention, these signal-strength
measurements are also used, in conjunction with a map or database
that correlates locations to signal strength, to estimate the
location of the wireless terminal.
In general, more signal-strength measurements provide a better
estimate of the location of the wireless terminal than fewer
signal-strength measurements, and, therefore, the acquisition of
additional signal-strength measurements is typically advantageous.
One way of acquiring an additional signal-strength measurement is
to actually physically measure a signal at the wireless terminal,
but most legacy terminals are not equipped to measure and report on
an arbitrary number of signals.
Another way of acquiring a "signal-strength measurement" is by
inference or deduction based on other information, and this is what
the illustrative embodiment does.
In particular, the illustrative embodiment deduces the signal
strength of one or more base stations' control channels at the
wireless terminal based on the principal of reciprocity, whether or
not the wireless terminal can actually receive the base stations'
control channels but so long as the base station can receive and
measure an uplink traffic channel signal from the wireless
terminal. This is accomplished in accordance with the principal of
reciprocity. The principal of reciprocity states that the
attenuation of a signal transmitted from Point A to Point B is the
same as that for a signal that is transmitted from Point B to Point
A.
In other words, the signal strength of the base station's control
channel signal at the wireless terminal, R.sub.D, can be deduced
from the strength at which the control channel signal is
transmitted by the base station, T.sub.D, and the attenuation of
that signal between the base station and the wireless terminal,
A.sub.D, by the function: R.sub.D=T.sub.D-A.sub.D (Eq. 1)
The principal of reciprocity indicates that the attenuation of the
signal between the base station and the wireless terminal, A.sub.D,
equals the attenuation of that signal between the wireless terminal
and the base station, A.sub.U, as represented by Equation 2:
A.sub.D=A.sub.U (Eq. 2)
The attenuation of the signal between the wireless terminal and the
base station, A.sub.U, is equal to the strength at which the signal
is transmitted by the wireless terminal, T.sub.U, minus the signal
strength of the signal as measured by the base station, R.sub.U, as
represented by Equation 3: A.sub.U=T.sub.U-R.sub.U (Eq. 3)
By substituting Equation 3 into Equation 2 and Equation 2 into
Equation 1, the signal strength of the base station's control
channel signal at the wireless terminal, R.sub.D, can be deduced
from the strength at which the control channel signal is
transmitted by the base station, T.sub.D, the strength at which the
signal is transmitted by the wireless terminal, T.sub.U, and the
signal strength of the uplink traffic channel signal from the
wireless terminal as measured by the base station, R.sub.U, as
represented by Equation 4: R.sub.D=T.sub.D-(T.sub.U-R.sub.U) (Eq.
4)
The deduced signal-strength measurements can then used--alone or in
combination with the empirical signal-strength measurements--to
estimate the location of the wireless terminal.
The illustrative embodiment comprises: receiving a signal-strength
measurement for a first downlink signal that is received by a
wireless terminal and a signal-strength measurement for an uplink
signal that is transmitted by the wireless terminal; and estimating
the location of the wireless terminal based on the signal-strength
measurement for the first downlink signal and on the
signal-strength measurement for the uplink signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a map of a portion of a wireless telecommunications
system in the prior art.
FIG. 2 depicts a map of the illustrative embodiment of the present
invention.
FIG. 3 depicts a block diagram of the salient components of
location system 212.
FIG. 4 depicts a broad overview of the salient operations performed
by the illustrative embodiment in ascertaining the location of
wireless terminal 201 in geographic region 200.
FIG. 5 depicts a flowchart of the salient operations performed in
operation 401.
FIG. 6 depicts a map of how geographic region 200 is partitioned
into 500 locations in accordance with the illustrative embodiment
of the present invention.
FIG. 7a depicts a graph that shows that the signal strength of an
electromagnetic signal decreases, in general, as a function of the
distance from the transmitter and in an environment with no radio
frequency obstacles.
FIG. 7b depicts a graph that shows that the signal strength of an
electromagnetic signal decreases, in general, as a function of the
distance from the transmitter and in an environment with two radio
frequency obstacles.
FIG. 8 depicts a map of the signal-strength measurements of the
signal radiated from base station 202-1 at each location in
geographic region 200.
FIG. 9 depicts a map of the signal-strength measurements of the
signal radiated from base station 202-2 at each location in
geographic region 200.
FIG. 10 depicts a map of the signal-strength measurements of the
signal radiated from base station 202-3 at each location in
geographic region 200.
FIG. 11 depicts a flowchart of the salient operations performed in
operation 402
FIG. 12 depicts a flowchart of the salient operations performed in
operation 1102.
FIG. 13 depicts a flowchart of the salient operations performed in
operation 1103.
FIG. 14 depicts a flowchart of the salient operations performed in
operation 404.
DETAILED DESCRIPTION
FIG. 2 depicts a map of the illustrative embodiment of the present
invention, which comprises: wireless switching center 211, location
system 212, base stations 202-1, 202-2, and 202-3, and wireless
terminal 201, which are interconnected as shown. The illustrative
embodiment provides wireless telecommunications service to most of
geographic region 200, in well-known fashion, and is also capable
of estimating the location of wireless terminal 201 within
geographic region 200.
The illustrative embodiment operates in accordance with the Global
System for Mobile Communications (formerly known as the Groupe
Speciale Mobile), which is ubiquitously known as "GSM." After
reading this disclosure, however, it will be clear to those skilled
in the art how to make and use embodiments of the present invention
that operate in accordance with other protocols, such as the
Universal Mobile Telephone System ("UMTS"), CDMA-2000, and IS-136
TDMA.
Wireless switching center 211 is a switching center as is
well-known to those skilled in the art in most respects but is
different in that it is capable of communicating with location
system 212 in the manner described below. After reading this
disclosure, it will be clear to those skilled in the art how to
make and use wireless switching center 211. It will also be clear
to those skilled in the art that a wireless switching center is
also known by other names, such as a mobile switching center, a
mobile telephone switching office, etc.
The illustrative embodiment comprises one wireless switching
system, but after reading this specification it will be clear to
those skilled in the art how to make and use embodiments of the
present invention that use two or more systems to obtain
signal-strength measurements. Typically, this is useful when a
wireless terminal is near the boundary of one or more systems. When
two or more systems are used to obtain signal-strength
measurements, one wireless switching center can use the IS-41
protocol messages HandoffMeasurementRequest and
HandoffMeasurementRequest2 to elicit signal-strength measurements
from another.
Base stations 202-1, 202-2, and 202-3 are well-known to those
skilled in the art and communicate with wireless switching center
211 through cables and other equipment (e.g., base station
controllers, etc.) that are not shown in FIG. 2. As depicted in
FIG. 2, wireless terminal 201 is serviced by base station 202-2.
Although the illustrative embodiment comprises three base stations,
it will be clear to those skilled in the art how to make and use
embodiments of the present invention that comprise any number of
base stations.
Each of base stations 202-1, 202-2, and 203-3 is capable, in
well-known fashion, of: i. measuring the signal strength of an
uplink traffic channel signal as transmitted by wireless terminal
201 and reporting that measurement to wireless switching center
211; and ii. informing wireless switching center 211 of the
strength at which it transmits the control channel. The first of
these capabilities is performed, for example, by the Digital Locate
Radio in IS-136 TDMA systems. It will be clear to those skilled in
the art how to make and use alternative embodiments of the present
invention in which some of the base stations measure another uplink
signal, such as an uplink signaling channel signal, etc.
In accordance with the illustrative embodiment, base station 202-1
receives and measures the signal strength of the uplink signal S
(depicted as a dashed zig-zag arrow in FIG. 2) from wireless
terminal 201. Base station 202-1 is in the vicinity of wireless
terminal 201, but base station 202-1 is neither (1) the serving
base station for wireless terminal 201 nor (2) on wireless terminal
201's list of neighboring base stations. Although the illustrative
embodiment depicts only one base station in the vicinity of
wireless terminal 201 for receiving and measuring the strength of
the uplink signal S and that is neither (1) the serving base
station nor (2) on wireless terminal 201's list of neighboring base
stations, it will be clear to those skilled in the art how to make
and use embodiments of the present invention that have any number
of such base stations.
In accordance with the illustrative embodiment, base station 202-2
is the serving base station for wireless terminal 201 and,
therefore, base station 202-2 receives and measures the signal
strength of the uplink signal S (depicted as a dashed zig-zag arrow
in FIG. 2) from wireless terminal 201. In accordance with the
illustrative embodiment, wireless terminal 201 does not measure the
signal strength of the control channel from base station 202-2. In
accordance with the illustrative embodiment, wireless terminal 201
receives a downlink traffic channel (depicted as a solid zig-zag
arrow in FIG. 2) from base station 202-2. Although the illustrative
embodiment depicts only one serving base station for wireless
terminal 201, it will be clear to those skilled in the art how to
make and use embodiments of the present invention that have one or
more serving base stations (e.g., in IS-95 Code-Division Multiple
Access systems, etc.).
In accordance with the illustrative embodiment, base station 202-3
is on wireless terminal 201's list of neighboring base stations,
and, therefore, wireless terminal 201 receives and measures the
signal strength of the control channel (depicted as a dotted
zig-zag arrow in FIG. 2) from base station 202-3. Although the
illustrative embodiment depicts only one base station on wireless
terminal 201's list of neighboring base stations, it will be clear
to those skilled in the art how to make and use embodiments of the
present invention in which a wireless terminal has any number of
base stations in it's list of neighboring base stations.
Base station 202-3 is in the vicinity of wireless terminal 201,
and, therefore, base station 202-3 receives and measures the signal
strength of the uplink signal S (depicted as a dashed zig-zag arrow
in FIG. 2) from wireless terminal 201. This is not a necessary
consequence of base station 202-3 being on wireless terminal 201's
list of neighboring base stations, but is merely a result of the
proximity of base station 202-3 and wireless terminal 201. In some
embodiments of the present invention a base station on a wireless
terminal's list of neighboring base stations will be able to
receive and measure the signal strength of the uplink signal S from
wireless terminal 201 and in some cases it will not. It will be
clear to those skilled in the art how to make and use embodiments
of the present invention in which some, none, or all of the base
stations on a wireless terminal's list of neighboring base stations
are able to receive and measure the signal strength of the uplink
signal S from wireless terminal 201.
Wireless terminal 201 is a standard GSM wireless terminal as it is
currently manufactured and used throughout the world. Wireless
terminal 201 is equipped, in well-known fashion, with the hardware
and software necessary to measure and report to wireless switching
center 211 on the signal strength of signals from the base stations
that are on its list of neighboring base stations.
A GSM wireless terminal, such as wireless terminal 201, is capable
of reporting the signal strength of a signal as one of 64 levels
between -47 dBm and -110 dBm. Any signal stronger than -47 dBm is
reported as -47 dBm, and any signal weaker than -110 dBm is
reported as -110 dBm.
In accordance with the illustrative embodiment of the present
invention all of the specific portions of the radio frequency
spectrum fall within the same band that wireless terminal 201 uses
to communicate with base stations 202-1, 202-2, and 202-3. In some
alternative embodiments of the present invention, however, some or
all of the specific portions of the radio frequency spectrum are
outside the band that wireless terminal 201 uses to communicate
with base stations 202-1, 202-2, and 202-3. In any case, it will be
clear to those skilled in the art how to make and use wireless
terminal 201.
Location system 212 is a computer system that is capable of
estimating the location of wireless terminal 201, as described in
detail below. Although the illustrative embodiment depicts location
system 212 as estimating the location of only one wireless
terminal, it will be clear to those skilled in the art that
location system 212 is capable of estimating the location of any
number of wireless terminals serviced by wireless switching center
211.
Furthermore, although location system 212 is depicted in FIG. 2 as
being distinct from wireless switching center 211, this is done
principally to highlight the distinction between the functions
performed by wireless switching center 211 and the functions
performed by location system 212. In other words, it will be clear
to those skilled in the art how to make and use embodiments of the
present invention in which location system 212 resides within or
without wireless switching center 211 or is a fully-integrated part
of wireless switching center 211.
Furthermore, although--again for pedagogical purposes--wireless
switching center 211, location system 212, and base stations 202-1,
202-2, and 202-3 are depicted in FIG. 2 as being within geographic
region 200 (i.e., the region of candidate locations for wireless
terminal 201), this is not necessarily so, and it will be clear to
those skilled in the art how to make and use embodiments of the
present invention in which some or all of these pieces of equipment
are not within the region of location estimation.
FIG. 3 depicts a block diagram of the salient components of
location system 212 in accordance with the illustrative
embodiment.
As shown in FIG. 3, location system 212 comprises: processor 301,
signal-strength database 302, receiver 303, and transmitter 304,
which are interconnected as shown.
Receiver 303 receives information from wireless switching center
211, as disclosed below and with respect to FIG. 4, and forwards
this information to processor 302.
Processor 301 is a general-purpose processor as is well-known in
the art that is capable of performing the operations described
below and with respect to FIG. 4. Processor 302 receives input from
receiver 303 and sends output to transmitter 304 in well-known
fashion.
Signal-strength database 302 is a non-volatile memory that stores
signal-strength measurements as described below and with respect to
FIG. 4.
Transmitter 304 receives output from processor 301 and transmits
this output to wireless switching center 211 in well-known
fashion.
Overview--FIG. 4 depicts a broad overview of the salient operations
performed by the illustrative embodiment in ascertaining the
location of wireless terminal 201 in geographic region 200. In
summary, the tasks performed by the illustrative embodiment can be
grouped for ease of understanding into five operations: i. the
population of signal-strength database 302, ii. the querying of the
wireless switching center 211 to provide uplink and downlink
signal-strength measurements, iii. the deduction of the signal
strengths at the wireless terminal for downlink signals whose
signal strength was not empirically measured at the wireless
terminal, iv. the estimation of the location of wireless terminal
201 based on measured downlink and deduced downlink signal
strengths, and v. the use of the estimated location of wireless
terminal 201. The details of each of these operations are described
briefly below and in detail afterwards with respect to FIGS. 5
though 14.
At operation 401, signal-strength database 302 associates each
location within geographic region 200 with a tuple of
signal-strength measurements for specific signals for that
location. Operation 401 is generally complex and potentially
expensive, and it is, therefore, preferably performed only
occasionally. The details of operation 401 are described in detail
below and with respect to FIG. 5.
At operation 402, location system 212 queries wireless switching
center 211 to learn: i. the empirical signal-strength measurements
for one or more control channel signals as received by wireless
terminal 211 and the corresponding transmit power of those control
channel signals; and ii. one or more empirical signal-strength
measurements for the uplink signal S as transmitted by wireless
terminal 201 and the corresponding transmit power of signal S.
The query is not for specific data but for whatever data wireless
switching center 211 can provide. In accordance with the
illustrative embodiment, location system 212 receives the following
data (which is summarized in Table 2): i. empirical signal-strength
measurements of the (downlink) control channels transmitted by base
station 202-3, as received by wireless terminal 201, which is
designated as R.sub.D(3), respectively; ii. the transmit power of
(uplink) signal S transmitted by wireless terminal 201, T.sub.U, at
substantially the same moment that wireless terminal 201 makes
measurement R.sub.D(3); iii. the empirical signal-strength
measurements of (uplink) signal S as received by base stations
201-1, 202-2, and 202-3, which are T.sub.U(1), T.sub.U(2), and
T.sub.U(3), respectively, at substantially the same moment that
wireless terminal 201 makes measurement R.sub.D(3); and iv. the
transmit power of the (downlink) control channel transmitted by
base stations 201-1, 202-2, and 202-3, which are T.sub.D(1),
T.sub.D(2), and T.sub.D(3), respectively, at substantially the same
moment that wireless terminal 201 makes measurement R.sub.D(3).
T.sub.D(1), T.sub.D(2), and T.sub.D(3) are needed along with
T.sub.U to deduce R.sub.D(1), R.sub.D(2), and R.sub.D(3). Although
a value for R.sub.D(3) was determined empirically and directly by
wireless terminal 201, for reasons given below, the deduced value
of R.sub.D(3) might be preferred.
TABLE-US-00002 TABLE 2 Signal-Strength Measurements Received by
Location System 212 In Accordance with Illustrative Embodiment i
R.sub.D(i) R.sub.U(i) T.sub.D(i) Base station 202-1 1 -- R.sub.U(1)
T.sub.D(1) Base station 202-2 2 -- R.sub.U(2) T.sub.D(2) Base
station 202-3 3 R.sub.D(3) R.sub.U(3) T.sub.D(3)
In accordance with the illustrative embodiment, wireless terminal
201 periodically or sporadically provides R.sub.D(3) and T.sub.U to
wireless switching center 211 in well-known fashion, and the
measurements are forwarded to location system 212. The details of
operation 402 are described in detail below and with respect to
FIGS. 11, 12, and 13.
At operation 403, location system 212 deduces the values of
R.sub.D(i) based on T.sub.U, R.sub.U(i), and T.sub.D(i). In
particular, location system 212 computes the uplink attenuation
A.sub.U(i) of signal S along path i in accordance with Equation 4:
R.sub.D(i)=T.sub.D(i)-(T.sub.U-R.sub.U(i)) (Eq. 4) When R.sub.D(i)
and R.sub.U(i) are at different frequencies, as in, for example, a
frequency-division duplexed system, the effects of fast fading
(i.e., Rayleigh fading) must be removed from R.sub.U(i) to ensure
that the deduced value of R.sub.D(i) is independent of fast fading
at the frequency of R.sub.U(i). As is well known in the art, the
effects of fast fading can be removed from R.sub.U(i) through
well-known filtering techniques. The details of operation 403 are
described in detail below and with respect to FIG. 12.
At operation 404, location system 212 estimates the location of
wireless terminal 201 based on all of the downlink signal-strength
measurements (i.e., the measured signal-strength measurements,
R.sub.1, . . . R.sub.n-1, and the deduced signal-strength
measurements, R.sub.D(1), . . . R.sub.D(3)), and a map or database
that correlates locations to signal-strength measurements.
In some cases, location system 212 will receive an empirical
signal-strength measurement for a downlink signal and will be able
to deduce the signal strength of that same downlink signal. In this
case, the deduced value should be used instead of the empirical
measurement because the accuracy of the uplink signal-strength
measurement is typically better than the accuracy of the downlink
signal-strength measurement. It will be clear to those skilled in
the art, however, after reading this specification, how to make and
use embodiments of the present invention in which one or more
empirical measurements are used instead of deduced values for the
measurements.
The details of operation 404 are described in detail below and with
respect to FIG. 14.
At operation 405, location system 212 transmits the location
estimated in operation 405 to an entity (not shown) for use in an
application. It is well known to those skilled in the art how to
use the estimated location of a wireless terminal in an
application.
At this point, operations 401 through 404 are described in
detail.
Population of Signal-Strength Database 302--FIG. 5 depicts a
flowchart of the salient operations performed in operation 401.
At task 501, geographic region 200 is partitioned into a plurality
of tessellated locations. Geographic region 200 is rectangular and
comprises 5,525 square arc-seconds, which near the equator equals
almost 5 square kilometers (see FIG. 6). After reading this
specification, it will be clear to those skilled in the art how to
make and use embodiments of the present invention that operate with
geographic regions of any size and shape.
In accordance with the illustrative embodiment of the present
invention, geographic region 200 is partitioned into a grid of 221
square locations that are designated location x.sub.1, y.sub.1
through location x.sub.17, y.sub.13. The number of locations into
which geographic location 200 is partitioned is arbitrary, subject
to the considerations described below. In accordance with the
illustrative embodiment, each location is an area of approximately
5 arc-seconds in length by 5 arc-seconds in height. Five
arc-seconds near the equator equals approximately 150 meters.
The size of the locations defines the highest resolution with which
the illustrative embodiment can locate a wireless terminal. In
other words, the illustrative embodiment can only estimate the
location of a wireless terminal to within one location (i.e., 5 by
5 arc-seconds in the illustrative embodiment). If greater
resolution is desired, for example 1 arc-second resolution, then
geographic region 200 would need to be partitioned into 1
arc-second by 1 arc-second locations. If geographic region 200 were
partitioned into 1 arc-second by 1 arc-second locations, there
would be 5,525 squares, which is considerably more than the 221
used in the illustrative embodiment. Although the ostensibly higher
resolution of 1 arc-second versus 5 arc-seconds is advantageous,
there are considerable disadvantages to a large number of
locations.
The number of locations to partition geographic region 200 into is
based on three factors. First, as the size of each location goes
down, the resolution of the embodiment increases. Second, as the
size of each location decreases, the number of locations in a
region increases, and, consequently, the computational complexity
of operation 404 increases quickly. Third, each location must be
large enough so that it has (at least slightly) different
signal-strength characteristics than its adjacent areas. This is
because the illustrative embodiment might--but won't
necessarily--have difficulty distinguishing between adjacent
locations that have the same signal-strength characteristics. It
will be clear to those skilled in the art how to consider these
three factors when deciding how to partition a geographic
region.
At task 502, the signal-strength measurements for a signal from
each base station are determined at each location in geographic
region. In accordance with the illustrative embodiment, the signal
used from each base station is the control channel because it is
broadcast at a constant power and because wireless terminal 201 can
distinguish it from every other control channel, if it can decode
its BSIC (for GSM networks).
Because there are three base stations in the illustrative
embodiment, each with one control channel, a tuple of three
signal-strength measurements at each location must be
determined.
In general, the signal strength of an electromagnetic signal
decreases as a function of the distance from the transmitter, as is
depicted in FIG. 7a, but the topography of the region and the
presence of buildings, trees, and other radio-frequency obstacles
severely alters this generalization, as is depicted in FIG. 7b.
In accordance with the illustrative embodiment, the tuple of three
signal-strength measurements for each location are determined
through a combination of: (i) a theoretical radio-frequency
propagation model, and (ii) empirical signal-strength measurements.
It will be clear to those skilled in the art how to accomplish
this.
For example, one well-known modeling technique for outdoor
radio-frequency signal propagation is adapted from the power-law
decay model. The power-law decay model assumes that the base
station's antenna is high above the ground and that there is
line-of-sight propagation to the wireless terminal. In this case,
the mean signal-strength, P, received at the wireless terminal
decays in inverse proportion to the square of the distance from the
transmitter,
.varies..times. ##EQU00001## up to some break-point. Beyond that
breakpoint, the mean power at the wireless terminal decays in
inverse proportion to the fourth power of the distance from the
transmitter:
.varies..times. ##EQU00002## The location of the break-point is
determined through empirical signal-strength measurements as the
location at which the ground bounce signal interferes with the
line-of-sight signal.
In accordance with another well-known model, the signal-strength
measurements at each location are determined by taking empirical
measurements at various locations and by interpolating for the
locations in between the sampled locations. This method is
advantageous in that it does not require many empirical
measurements to be made, but it is less accurate than taking
measurements at every location.
It will be clear to those skilled in the art how to determine the
signal-strength measurements for each location in the geographic
region whether through: (i) theoretical radio-frequency propagation
models, or (ii) empirical signal-strength measurements, or (iii)
any combination of i and ii.
In accordance with the illustrative embodiment, FIG. 8 depicts the
signal strength of the signal from base station 202-1 (hereinafter
referred to as "Signal 1") at each location in geographic region
200. In general, Signal 1 is stronger near base station 202-1 and
weaker far away from base station 202-1.
In accordance with the illustrative embodiment, FIG. 9 depicts the
signal strength of the signal from base station 202-2 (hereinafter
referred to as "Signal 2") at each location in geographic region
200. Like Signal 1, Signal 2 is stronger near base station 202-2
and weaker far away from base station 202-2.
In accordance with the illustrative embodiment, FIG. 10 depicts the
signal strength of the signal from base station 202-3 (hereinafter
referred to as "Signal 3") at each location in geographic region
200. Like Signals 1 and 2, Signal 3 is stronger near base station
202-3 and weaker far away from base station 202-3.
When the signal-strength tuples for each location in geographic
region 200 have been determined, they are stored in signal-strength
database in a data structure that associates each location with the
tuple for that location. The data structure is then stored in
signal-strength database 302.
TABLE-US-00003 TABLE 3 Signal-Strength Database Signal-Strength
Tuple Strength of Strength of Strength of Location Signal 1 Signal
2 Signal 3 x.sub.1, y.sub.1 -115 -115 -115 x.sub.2, y.sub.1 -115
-115 -111 . . . . . . . . . . . . X.sub.7, y.sub.7 -45 -51 -49
X.sub.8, y.sub.7 -46 -52 -55 X.sub.9, y.sub.7 -50 -49 -62 . . . . .
. . . . . . . X.sub.16, y.sub.13 -115 -96 -115 X.sub.17, y.sub.13
-115 -105 -115
Table 3 depicts a portion of an illustrative data structure for
associating each location with the signal-strength tuple for that
location.
The three signal-strength measurements in a row of table 1
constitute a "tuple" or non-empty set of ordered elements. For
example, the signal-strength tuple at Location x.sub.7, y.sub.7 are
the 3-tuple {-45, -51, -49}. In general, the illustrative
embodiment of the present invention estimates the location of a
wireless terminal by pattern matching the signal-strength
measurements by the wireless terminal at a location against the
signal-strength tuples in signal-strength database 302. This
process is described in detail below and with respect to operation
402.
From task 502, control passes to operation 402 in FIG. 4.
Query of Transmit Strength and Signal-Strength Measurements from
Wireless Switching Center--FIG. 11 depicts a flowchart of the
salient operations performed in operation 402.
At task 1101, location system 212 transmits a request to wireless
switching center 211 to provide: i. the uplink transmit signal
strength for wireless terminal 201, Tu, ii. the downlink
signal-strength measurements from wireless terminal 201, R.sub.D(3)
for some or all of the control channels that wireless terminal 201
is able to receive, and iii. the uplink signal-strength
measurements at various base stations that can receive and measure
wireless terminal 201's uplink signal, R.sub.U(1) through
R.sub.U(3). The third of these can be requested from wireless
switching center 211 in the form of an IS-41
HandoffMeasurementRequest or HandoffMeasurementRequest2 message, in
well-known fashion.
At task 1102, wireless switching center 211 obtains the uplink
transmit signal strength for wireless terminal 201, T.sub.U, and
the downlink signal-strength measurements from wireless terminal
201, R.sub.D(3) for some or all of the control channels that
wireless terminal 201 is able to receive (e.g., base station 202-2
in FIG. 2, etc.) in well-known fashion. Task 1102 is described in
detail below and with respect to FIG. 12.
At task 1103, wireless switching center 211 obtains the uplink
signal-strength measurements, R.sub.U(1) through R.sub.U(3), in
well-known fashion. Task 1103 is described in detail below and with
respect to FIG. 13.
At task 1104, location system 212 receives from wireless switching
center 211: i. the uplink transmit signal strength for wireless
terminal 201, T.sub.U, ii. the downlink signal-strength
measurements from wireless terminal 201, R.sub.D(3) for some or all
of the control channels that wireless terminal 201 is able to
receive, and iii. the uplink signal-strength measurements at
various base stations that can receive and measure wireless
terminal 201's uplink signal, R.sub.U(1) through R.sub.U(3).
FIG. 12 depicts a flowchart of the salient operations performed in
task 1102.
At subtask 1201, wireless switching center 211 directs wireless
terminal 201, in well-known fashion, to (1) attempt to receive the
control channels for the base stations on its list of neighboring
base stations, (2) report back a signal-strength value for each
received control channel, and (3) report back the transmit strength
of a signal that it transmits.
At subtask 1202, wireless terminal 201 reports, in well-known
fashion, signal-strength measurements R.sub.1 . . . R.sub.n-1 for
some or all of the control channels that it is able to receive to
its serving cell's base station (e.g., base station 202-2 in FIG.
2, etc.).
At subtask 1203, wireless terminal 201 reports to its serving
cell's base station, in well-known fashion, the transmit strength
of a signal S transmitted by wireless terminal 201, R.sub.U. As is
well-known in the art, wireless terminal 201 regularly transmits
signals, and any of these signals can be used as "signal S" with
respect to tasks 1103 and 1202, disclosed below.
At subtask 1204, the base station forwards (i) the signal-strength
measurements received at task 1102, and (ii) the transmit strength
received at task 1103, to wireless switching center 211 in
well-known fashion.
At subtask 1205, wireless switching center 211 forwards (i) the
signal-strength measurements received at task 1102, and (ii) the
transmit strength received at task 1103, to location system 212 in
well-known fashion.
It will be clear to those skilled in the art how to make and use
embodiments of the present invention that perform operation 402.
From task 1205, control passes to operation 1104 in FIG. 11.
Receipt of (iii) Transmit Strength of control channels and (iv)
Signal-Strength Measurement of Signal S--FIG. 13 depicts a
flowchart of the salient operations performed in operation
1103.
At subtask 1301, wireless switching center 211 directs the base
stations in the vicinity of wireless terminal 201 to measure and
report the signal-strength of the uplink signal from wireless
terminal 201, in well-known fashion.
At subtask 1302, each of the base stations that are able to measure
the signal strength of the uplink signal from wireless terminal 201
reports to wireless switching center 211 the measured signal
strength, in well-known fashion.
At subtask 1303, each of the base stations that reported a value in
subtask 1302 reports to wireless switching center 211 the transmit
strength of the control channel that it transmits, in well-known
fashion.
At subtask 1304, wireless switching center 211 forwards the
signal-strength measurements received in subtask 1302 and the
control channel transmit strengths received in subtask 1303 to
location system 212 in well-known fashion.
It will be clear to those skilled in the art how to make and use
embodiments of the present invention that perform operation 1103.
From task 1204, control passes to operation 1104 in FIG. 11.
Estimation of the Location of Wireless Terminal 201--FIG. 14
depicts a flowchart of the salient operations performed in
operation 404. For pedagogical purposes, operation 404 as depicted
in FIG. 14 is described three times. First, operation 404 is
described in the abstract with a focus on describing its underlying
theory. Next, operation 404 is described as it is applied to the
first report, and finally, operation 404 is described as it is
applied to the second report.
In accordance with the first report, the signal strength of Signal
1, R.sub.D(1), is deduced and equals -98, the signal strength of
Signal 2, R.sub.D(2), is deduced, and equals -64, and the signal
strength of Signal 3, R.sub.D(3), is empirically measured, and
equals -51.
In accordance with the second report, the signal strength of Signal
1, R.sub.D(1), is deduced and equals -98, the signal strength of
Signal 2, R.sub.D(2), is deduced, and equals -64, and the signal
strength of Signal 3, R.sub.D(3), is empirically measured, and
equals -50.
Estimation in General--Subtask 1401 begins with 211
(17.times.13=211) candidate locations that must be considered as
the location for wireless terminal 201, and, therefore, 211
signal-strength tuples (i.e., the 211 tuples in signal-strength
database 302) that must be processed. Subtasks 1402 through 1405
can be computationally intense, and the computational burden
increases with the number of candidate locations that must be
considered. Therefore, location system 212 attempts, at subtask
1401, to reduce the number of candidate locations that must be
processed in subtasks 1402 through 1405.
To reduce the number of candidate locations that must be processed
in subtasks 1402 through 1405, location system 212 uses the
following observation. When a downlink signal is reported with a
maximum signal strength (i.e., "-47" in the illustrative
embodiment), location system 212 can reasonably eliminate from
consideration as a candidate location every location where the
signal-strength measurement for that signal is below the maximum
(minus a factor for measurement errors and systematic bias). In
other words, when a signal is reported with a maximum signal
strength, location system 212 can restrict consideration in
subtasks 1402 through 1405 to those candidate locations where
signal-strength database 302 predicts the signal strength to be
greater than or equal to the maximum reportable value (minus a
factor for measurement errors and systematic bias). In accordance
with the illustrative embodiment, the factor for measurement errors
and systematic bias is 3 dBm, and, therefore when a downlink signal
is reported with -47, location system 212 can restrict
consideration in subtasks 1402 through 1405 to those candidate
locations where signal-strength database 302 predicts the signal
strength to be greater than or equal to -50 dBm. It will be clear
to those skilled in the art how to determine and use other factors
for measurement errors and systemic bias.
At subtask 1402, location system 212 computes the signal-strength
differentials for all of the downlink signal-strength measurements
(i.e., both the empirical signal-strength measurements and the
deduced signal-strength measurements) that are not at the maximum
signal strength. In particular, for n unique downlink
signal-strength measurements that are not at the maximum signal
strength, n-1 signal-strength differentials are computed where:
.DELTA.S.sub.k=S.sub.k-S.sub.1 (Eq. 7) for k=2, 3, . . . n, wherein
.DELTA.S.sub.k is the kth signal-strength differential, S.sub.k is
the downlink signal strength of Signal k, and S.sub.1 is the
downlink signal strength of Signal 1. When m of the downlink
signals is at the maximum signal strength (i.e., -47 dBm), then
n-m-1 (Eq. 8) pair-wise differentials for the remaining n-m signals
are computed, in well-known fashion. At the end of subtask 1402,
location system 212 will have computed n-m-1 pair-wise
differentials, .DELTA.S.sub.2 through .DELTA.S.sub.n-m.
At subtask 1403, location system 212 computes the signal-strength
differentials for only those locations that were not eliminated
from consideration in task 1201.
Furthermore, location system 212 only computes the signal-strength
differentials corresponding to the differentials computed in
subtask 1402; the idea, of course, being to ensure that "apples are
compared with apples." In particular, for n reported signals that
are not at the maximum signal strength, n-1 signal-strength
differentials are computed where:
.DELTA.R.sub.k,x,y=R.sub.k,x,y-R.sub.1,x,y (Eq. 9) for k=2, 3, . .
. n, wherein .DELTA.R.sub.k,x,y is the kth signal-strength
differential for location x,y, R.sub.k,x,y is the signal-strength
of Signal k at location x,y in signal-strength database 302, and
R.sub.1,x,y is the reported signal strength of Signal 1 at location
x,y in signal-strength database 302.
At the end of subtask 1403, location system 212 will have computed
n-m-1 pair-wise differentials, .DELTA.R.sub.2,x,y through
.DELTA.R.sub.n-m,x,y, corresponding to the pair-wise differentials
computed in subtask 1403, for all the candidate locations.
At subtask 1404, location system 212 compares the signal-strength
differentials computed in subtask 1402, .DELTA.S.sub.2 through
.DELTA.S.sub.n-m, to the signal-strength differentials in subtask
1403, .DELTA.R.sub.2,x,y through .DELTA.R.sub.n-m,x,y, to generate
a probability distribution that indicates the goodness of fit
between the signal-strength differentials computed from the values
received in operations 402 and 403 to the signal-strength
differentials computed from the tuples in signal-strength database
302. To accomplish this, the Euclidean norm at each of the i
candidate locations is computed for the signal-strength
differentials computed from the values received in operations 402
and 403 and each of the signal-strength differentials computed from
the tuples in signal-strength database 302. This is described in
Equation 10. v.sub.x,y= {square root over
(.SIGMA..sub.2.sup.n(.DELTA.R.sub.k,x,y-.DELTA.S.sub.k).sup.2)}
(Eq. 10) wherein v.sub.x,y is the Euclidean norm between the
signal-strength tuple for location x,y in signal-strength database
302 in comparison to the signal-strength differentials computed
from the values received in operations 402 and 403.
Next, the Euclidean norms computed in Equation 4 are turned into
un-normalized probabilities by Equation 11:
e.times..tau..times. ##EQU00003## where .tau..sup.2 represents the
amount of uncertainty in both .DELTA.S.sub.k and
.DELTA.R.sub.k,x,y.
And finally, the values of P.sub.x,y are normalized to generate the
probability distribution for the location of wireless terminal 201
in geographic region 200.
At subtask 1405, location system 212 estimates the location of
wireless terminal 201 based on the probability distribution
generated in subtask 1404. In accordance with the illustrative
embodiment, location system 212 estimates the location of wireless
terminal based on the geometric mean of the probability
distribution generated in subtask 1404. After reading this
specification, however, it will be clear to those skilled in the
art how to make and use embodiments of the present invention that
estimate the location of wireless terminal 201 based on another
function of the probability distribution generated in subtask 1404,
such as the maximum likelihood function.
From subtask 1405, control passes to operation 404 in FIG. 4.
Estimation as Applied to First Report (Signal 1=-98. Signal 2=-64,
and Signal 3=-51)--At subtask 1401, location system 212 cannot
eliminate any candidate locations from consideration based on the
fact that none of the reported signals is at the maximum reportable
value minus the factor for measurement errors and systematic bias
(i.e., 3 dBm). In other words, location system 212 cannot eliminate
any candidate signal from consideration because all of the signals
are at -51 dBm or less. Therefore, location system 212 must
consider all 221 candidate locations in subtasks 1402 through
1305.
At subtask 1402, location system 212 computes two (2)
signal-strength differentials for the first report in which
R.sub.1=Signal 1=-98, R.sub.2=Signal 2=-64, and R.sub.3=Signal
3=-43. In particular, .DELTA.R.sub.2 and .DELTA.R.sub.3 are
computed as depicted in Table 4.
TABLE-US-00004 TABLE 4 Signal-strength Differentials for Signal 1 =
-98, Signal 2 = -64, and Signal 3 = -43 k .DELTA.R.sub.k R.sub.k -
R.sub.1 2 34 -64 - (-98) 3 47 -51 - (-98)
At subtask 1403, location system 212 computes two (2)
signal-strength differentials for each of the 221 locations in
signal-strength database 302, as depicted in Table 5.
TABLE-US-00005 TABLE 5 Signal-strength Differentials for Each Tuple
in Signal-Strength Database 302 Location .DELTA.S.sub.2,x,y
.DELTA.S.sub.3,x,y x1, y1 -110 - (-110) = 0 -110 - (-110) = 0 x2,
y1 -110 - (-110) = 0 -111 - (-110) = -1 x3, y1 -110 - (-110) = 0
-97 - (-110) = 3 . . . . . . . . . x16, y13 -96 - (-110) = 14 -110
- (-110) = 0 x17, y13 -105 - (-110) = 5 -110 - (-110) = 0
At subtask 1404, location system 212 first computes the Euclidean
norm between the signal-strength differentials in Table 2 against
the signal-strength differentials for each location in Table 3 to
produce the norms shown in Table 6.
TABLE-US-00006 TABLE 6 Euclidean Norms for Each Location (First
Report) Location Vx,y x1, y1 64.66 x2, y1 63.81 x3, y1 62.13 . . .
. . . x16, y13 58.52 x17, y13 62.18
Next, the Euclidean norms in Table 6 are converted to unnormalized
probabilities, as described above, and then the unnormalized
probabilities are normalized, in well-known fashion, to produce the
probability distribution of the location of wireless terminal 201
at each of the 211 locations in geographic region 200.
Estimation As Applied to Second Report (Signal 1=-98. Signal 2=-64,
and Signal 3=-50)--At subtask 1401, location system 212 can
perfunctorily eliminate most of the candidate locations from
consideration because the reported signal strength of one of the
reported signals--Signal 3=-50 dBm--is greater than the maximum
reported value (-47 dBm) minus the factor for measurement errors
and systematic bias (3 dBm). In other words, location system 212
can eliminate from consideration any candidate location in which
S.sub.3 is not at least -50 dBm. Therefore, location system 212 can
restrict consideration in subtasks 1402 through 1305 to those
locations in signal-strength database 302 in which Signal 3 is
predicted to be -50 dBm or greater. As can be seen in FIG. 10,
there are only 13 locations (x8,y4; x9,y4; x10,y4; x7,y5; x8,y5;
x9,y5; x10,y5; x7,y6; x8,y6; x9,y6; x10,y6; x7,y7; x8,y7; x9,y7) at
which Signal 3 is predict stronger, and, therefore, location system
212 need only perform subtasks 1402 through 1305, in the
above-described fashion, on those 13 locations. By reducing the
number of candidate locations that need to be processed from 221 to
13, task 1401 has greatly reduced the computational complexity of
operation 403.
It is to be understood that the above-described embodiments are
merely illustrative of the present invention and that many
variations of the above-described embodiments can be devised by
those skilled in the art without departing from the scope of the
invention. It is therefore intended that such variations be
included within the scope of the following claims and their
equivalents.
* * * * *